For heart function examination, a cardiac output is measured by a right heart catheter technique which typically relies on thermodilution. In the right heart catheter technique, the catheter is introduced from a jugular vein, femoral vein, cubital vein or the like and advanced through the superior or inferior vena cava, right atrium and right ventricle until its distal end reaches the pulmonary artery.
The catheter is provided with an injection port and a thermistor as a temperature sensing element such that the injection port is located in the right atrium and the thermistor is located in the pulmonary artery when the catheter is set in place. The injection port through which a fluid having a higher or lower temperature than the blood is introduced is located proximal of the thermistor. The temperature of the fluid as discharged is diffused and diluted in the right atrium and right ventricle. The diluted temperature is detected by the thermistor located in the pulmonary artery. A cardiac output is determined from a change with time of the detected temperature, that is, a dilution curve according to the Stewart-Hamilton method.
The cardiac output measurement by the thermodilution method is, however, intermittent and not applicable to continuous cardiac output measurement. When it is desired to take a plurality of measurements, the total amount of fluid introduced is increased, which imposes an increased burden to the patient. Repetition of such operations can increase the risk of infection.
U.S. Pat. No. 4,841,981 (or Japanese Patent Application Kokai No. 207435/1987) discloses a catheter for the continuous measurement of a cardiac output. This catheter includes temperature sensors for thermodilution measurement of a cardiac output and for measurement of a velocity of blood flow. It provides continuous measurement of blood flow velocity, from which a cardiac output is continuously calculated.
A typical arrangement of the catheter is shown in FIGS. 9 and 10. The catheter generally designated at 1 includes an elongated catheter tube 2 defining four lumens therein and having distal and proximal ends. The catheter 1 includes a pressure port 6 at the distal end of the catheter tube 2 and a balloon 8 of soft elastomer attached thereto so as to entirely surround the catheter tube over a distance of several mm from its distal end. The catheter tube 2 is provided with a side port 7 formed in the tube side wall surrounded by the balloon 8 for passage of a gas such as carbon dioxide for inflating and deflating the balloon, a second thermistor 45 spaced 10 to 20 mm apart from the distal end, a first thermistor 41 spaced 10 to 15 mm apart from the second thermistor 45 toward the proximal end, and an injection port 5 disposed proximal of the thermistors 41 and 45 and spaced 12 to 40 cm apart from the distal end.
Since this catheter is for arterial use, that is, adapted to be left in the pulmonary artery by inserting from a superior or inferior limb vein, thermistors 41 and 45 are provided on a catheter distal portion which is located downstream of the injection port 5 in the direction of blood flow. If the catheter is for venous use, that is, adapted to be left in the pulmonary vein, thermistors 41 and 45 should be provided on a catheter proximal portion which is located downstream of the injection port 5 in the blood flow direction.
The pressure port 6, balloon side port 7, thermistors 41 and 45, and injection port 5 are in communication with four independent lumens, respectively. More particularly, the pressure port 6 is in communication with a lumen 36 for measuring the external pressure such as pulmonary arterial pressure, the balloon side port 7 in communication with a balloon lumen 37, the thermistors 41 and 45 in communication with a lumen 34, and the injection port 5 in communication with a lumen 35 for injecting indicator fluid.
At the proximal end 29 of the catheter tube 2, the thermistor lumen 34 is coupled to connectors 941 and 945 through a bifurcated section of tubing such that lead wires 43 and 47 extending from the thermistors 41 and 45 are electrically connected to the connectors 941 and 945 through the thermistor lumen 34 and the section of tubing. Also, the pressure measuring lumen 36, injecting lumen 35 and balloon lumen 37 are coupled to connectors 96, 95 and 97 through sections of tubing, respectively.
The first thermistor 41 serving as a temperature sensor is for thermodilution measurement of a cardiac output whereas the second thermistor 45 is a thermistor of direct or indirect heating type which is adapted to be heated by electricity conduction or by another heater and produces a heating temperature signal representative of its own temperature, thereby sensing a blood flow velocity.
The four lumens 34, 35, 36 and 37 are defined in the interior bore of the catheter tube 2 by dividing the bore by a crisscross partition 28 into four sectors disposed about the center as shown in FIG. 10. The measuring lumen 36 is disposed adjacent the thermistor lumen 34.
On clinical application, the cardiac output measuring catheter, which is adapted for continuous measurement, is typically used in measurement for a long period of time, during which continuous fluid infusion is often needed. More particularly, it is often necessary to continuously infuse a suitable fluid through the pressure port 6 at the catheter distal end for pressure measurement. If such fluid is passed through the pressure measuring lumen 36 which is disposed adjacent the thermistor lumen 34, then the thermistors 41 and 45 are thermally affected by the fluid. In principle, the second thermistor 45 which is of the heating type is generally heated by conduction of constant current and senses its own temperature at the same time. Blood flow velocity is determined from the temperatures the first and second thermistors 41 and 45 sense and the heating current. If the temperature in the thermistor lumen 34 is substantially affected by the temperature of the fluid passing through the pressure measuring lumen 36, there occurs an inconvenience that the heating temperature signal, blood temperature signal and the value of conducting current can change, failing in accurate detection of blood flow velocity. Such thermal influence will also occur when only the first thermistor 41 for measuring a cardiac output is used.
Furthermore, the thermistor lumen 34 is disposed contiguous to the measuring lumen 36 in the above-cited publication. With the partition structure disclosed therein and illustrated in FIG. 10, even if the pressure measuring lumen is changed to lumen 37 to separate the thermistor lumen 34 from the measuring lumen (37), there still remains a risk of failing to provide accurate measurement of blood flow velocity because the partition sections are contiguous.